Keep Digging: What lies beneath?

Professor Chris MacLeod has featured in a BBC 4 radio documentary to explore why we know so little about the Earth’s mantle and what lies beneath.

In the programme, host Mary-Ann Ochota asks why scientists have managed to send probes to interstellar space, but haven't yet managed to reach the Earth's mantle, which lies just a few miles beneath us.

Professor MacLeod was Co-Chief Scientist of an International Ocean Discovery Program (IODP) drilling mission in 2016 that had the aim of exploring how the lower ocean crust is generated, the nature of the 'Moho' – a seismological boundary within the Earth taken as the boundary between the crust and mantle – and how far seawater penetrates into the sub-seafloor. The ultimate goal of the project is to drill a 'Mohole': an ultra-deep borehole all the way into the Earth's mantle. He is one of the scientists championing the need for further exploration in the programme. Despite a low level of public interest historically, and therefore funding opportunities, the programme highlights why we should keep drilling.

More than half a century since the advent of the theory of plate tectonics our knowledge of the nature of oceanic basement, and of the Earth's mantle beneath, remains remarkably limited. Simple conceptual models for the structure of the ocean crust, based on theoretical frameworks and scant rock sampling, are being ever more questioned as each new piece of evidence emerges. For example, the finding of altered mantle samples on the seafloor, modified by extensive interaction with seawater, shows that ocean crust may be absent entirely, potentially over huge swathes of the planet's surface. Not only do we now call into question basic models for ocean crustal structure and nature of the Moho boundary but, because the altered mantle rocks contain water, carbon and – remarkably – microbes living within the solid rock, we are obliged to re-examine fundamental assumptions about Earth's water and carbon cycles, and estimates of global biomass.

“Keep Digging” was aired on 4th April 2017. Listen again via the radio iPlayer.